Photolithography plays a critical role in the art of printed circuit packaging. Photolithography is used to define in a thin film of photoresist those regions either from which copper is to be selectively etched to subtractively form circuitization, or selectively plated to additively form circuitization.
There are basically two types of photoresist: negative acting and positive acting. Positive photoresists are used extensively to fabricate silicon devices. They, however, perform poorly in high caustic environments and high temperatures. The negative resists, on the other hand, are used when the circuit lines are provided by additive plating of copper, in areas where copper is desired, i.e., electroless or electroless plus electroplating, rather than by etching of copper away from where it is not desired.
When a negative photoresist is selectively exposed to the particular radiation to which it is sensitive for an adequate period of time and then subjected to its developer, the areas of the resist which have not been exposed to radiation are removed by the developer, whereas the areas which have been exposed to radiation are hardened thereby by cross-linking and made more resistant to developer, relative to the unexposed regions. On the other hand the positive acting resists behave oppositely; the exposed regions are removed preferentially.
Photolithographic processes in packaging are described in Microelectronics Packaging Handbook, Pub. Van Nostrand Reinhold, New York, 1989, Tummala et al., eds. on pages 898-903, in Principles of Electronic Packaging, McGraw-Hill Book Company, New York, 1989, Seraphim et al., eds. in Chapter 12, pages 372-393 and in Scientific Encyclopedia, 6th Ed., Vol II, Pub. Van Nostrand Reinhold Company, New York, 1983, Considine et al., eds, pages 1877-1881, all of which are incorporated herein by reference for use as background.
In general negative-working resists include those photopolymerizable materials of the type described in U.S. Pat. No. 3,469,982, U.S. Pat. No. 4,273,857 and U.S. Pat. No. 4,293,635 and the photocrosslinkable species of the type disclosed in U.S. Pat. No. 3,526,504.
Included in the following are monomers which can be used either alone or in combination with others such as those in the conventional photoresists: t-butyl acrylate, 1, 5 pentanediol diacrylate, N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate, 1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethylene glycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycol diacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanediol diacrylate, 2,2-dimethylolpropane diacrylate, glycerol diacrylate, tripropylene glycol diacrylate, glycerol triacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyoxyethylated trimethylolpropane triacrylate and trimethacrylate and similar compounds as disclosed in U.S. Pat. No. 3,380,831, 2,2-di-(p-hydroxyphenyl)-propane diacrylate, pentaerythritol tetraacrylate, 2,2-di(p-hydrohyphenyl)-propane dimethacrylate, triethylene glycol diacrylate, polyoxyethyl-2,2-di(p-hydroxyphenyl)-propane dimethacrylate, di-(3-methacryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-methacryloxyethyl) ether of bisphenol-A, di-(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-acryloxyethyl) ether of bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-bisphenol-A, di-(2-methacryloxyethyl) ether of tetrachloro-bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrabromo-bisphenol-A, di-(2-methacryloxyethyl) ether of tetrabromo-bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of 1,4-butanediol, di-(3-methacryloxy-2-hydroxypropyl) ether of diphenolic acid, triethylene glycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate, ethylene glycol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 2,2,4-trimethyl-1, 3-pentanediol dimethacrylate, pentaerythritol trimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate, pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate, 1,5-pentanediol dimethacrylate, diallyl fumarate, styrene, 1,4-benzenediol dimethacrylate, 1,4-diisopropenyl benzene, and 1,3,5-triisopropenyl benzene.
In addition to the monomers mentioned above, the photoresist material can also contain one or more free radical-initiated and polymerizable species with molecular weight of at least about 300. Monomers of this type are an alkylene or a polyalkylene glycol diacrylate and those described in U.S. Pat. No. 2,927,022.
Free radical initiators which can be activated by actinic radiation which are thermally inactive at and below 185.degree. C. include the substituted or unsubstituted polynuclear quinones listed in the following: 9,10-anthraquinone, 1-chloroanthraquinone, 2-chloroanthraquinone, 2-methylanthraquinone, 2-tertbutylanthraquinone, octamethylanthraquinone, 1,4-naphthoquinone, 9,10-phenanthrequinone, 1,2-benzanthraquinone, 2,3-benzanthraquinone, 2-methyl-1, 4-naphthone, 2,3-dichloronaphthoquinone, 1,4-dimethylanthraquinone, 2,3-dimethylanthraquinone, 2-phenylanthraquinone, 2,3-diphenylanthraquinone, 2,3-diphenylanthraquinone, sodium salt of anthraquinone alpha-sulfonic acid, 3-chloro-2-methylanthraquinone, retenequinone, 7,8,9,10-tetrahydronaphthacenequinone, and 1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione.
Other useful photoinitiators, of which some may be thermally active at temperatures lower than 85.degree. C., are described in U.S. Pat. No. 2,760,863.
Dyes of a photoreducible nature and other reducing agents are described in U.S. Pat. Nos. 2,850,445; 2,875,047; 3,097,096; 3,074,974; 3,097,097; and 3,145,104 as well as dyes of the phenazine, oxazine and quinone clases; Michler's ketone, benzophenone, 2,4,5-triphenylimidazolyl dimers with hydrogen donors, and mixtures thereof as described in U.S. Pat. Nos. 3,427,161; 3,479,185 and 3,549,367 can be used as initiators. The cyclohexadienone compounds of U.S. Pat. No. 4,341,860 are also useful as initiators. In addition sensitizers described in U.S. Pat. No. 4,162,162 in combination with photoinitiators and photoinhibitors are useful.
Polymeric binders which can be used alone, or in combination with other of the same or other polymerizable monomers include the following: polyacrylate and alpha-alkyl polyacrylate esters, i.e., polymethyl methacrylate and polyethyl methacrylate; polyvinyl esters, i.e., polyvinyl acetate, polyvinyl acetate/acrylate, polyvinyl acetate/methacrylate and hydrolyzed polyvinyl acetate; ethylene/vinyl acetate copolymers; polystyrene polymers and copolymers, i.e., with maleic anhydride and esters; vinylidene chloride copolymers, i.e., vinylidene chloride/acrylonitrile; vinylidene chloride/methacrylate and vinylidene chloride/vinyl acetate copolymers; polyvinyl chloride and copolymers, i.e., polyvinyl chloride/acetate; saturated and unsaturated polyurethanes; synthetic rubbers, i.e., butadiene/acrylonitrile, acrylonitrile/butadiene/styrene, methacrylate/acrylonitrile/butadiene/styrene copolymers, 2-chlorobutadiene-1,3 polymers, chlorinated rubber, and styrene/butadiene/styrene, styrene/isoprene/styrene block copolymers; high molecular weight polyethylene oxides of polyglycols having average molecular weight from about 4,000 to 1,000,000 epoxides, i.e., containing acrylate or methacrylate groups; copolyesters; nylons or polyamides, i.e., N-methoxymethyl, polyhexamethylene adipamide; cellulose esters, i.e., cellulose acetate succinate and cellulose acetate butyrate; cellulose ethers, i.e., methyl cellulose, ethyl cellulose and benzyl cellulose; polycarbonates; polyvinyl acetal, i.e., polyvinyl butyral, polyvinyl formal; polyformaldehydes.
In addition to the polymeric binders listed above, particulate thickeners such as described in U.S. Pat. No. 3,754,920, i.e., silicas, clays, alumina, bentonites, kalonites, and the like can be used.
Where aqueous developing of the photoresist is desirable the binder should contain sufficient acidic or other functionalities to render the composition processable in the aqueous developer. Suitable aqueous-processable binders include those described in U.S. Pat. No. 3,458,311 and in U.S. Pat. No. 4,273,856. Polymers derived from an aminoalkyl acrylate or methacrylate, acidic film-forming comonomer and an alkyl or hydroxyalkyl acrylate such as those described in U.S. Pat. No. 4,293,635 can be included.
Normally, a thermal polymerization inhibitor will be present to increase the stability during storage of the photosensitive compositions. Such inhibitors are: p-methoxy-phenol, hydroquinone, alkyl and aryl-substituted hydroqinones and quinones, tert-butyl catechol, pyrogallol, copper resinate, naphthylamines, beta-napthol, cuprous chloride, 2,6-di-tert-butyl-p-cresol, phenothiazine, pyridine, nitrobenzene and dinitrobenzene, p-toluequinone and chloranil. Also useful for thermal polymerization inhibitors are the nitroso compositions described in U.S. Pat. No. 4,168,982.
Dyes and pigments may also be added to increase the visibility of the resist image. Any colorant used however, should be transparent to the actinic radiation used.
An example of such photosensitive compositions is described in Table I of U.S. Pat. No. 4,693,959.
In preparation of these formulations generally inert solvents are employed which are volatile at ordinary pressures. Examples include alcohols and ether alcohols, esters, aromatics, ketones, chlorinated hydrocarbons, aliphatic hydrocarbons, miscellaneous solvents such as dimethylsulfoxide, pyridine, tetrahydrofuran, dioxane, dicyanocyclobutane and 1-methyl-2-oxo-hexamethyleneimine, and mixtures of these solvents in various proportions as may be required to attain solutions. Antiblocking agents to prevent the coatings from adhering to the supporting films can also be included.
With some polymers, it is desirable to add a plasticizer, either solid or liquid, to give flexibility to the film or coating. Suitable plasticizers are described in U.S. Pat. No. 3,658,543. A preferred liquid plasticizer is nolylphenoxypoly(ethyleneoxy)ethanol. A preferred solid plasticizer is N-ethyl-p-toluenesulfonamide.
Photoimagable compositions are also utilized as solder masks in various industrial processes. In such application a photoimagable composition is used by applying the composition to printed circuit board and followed by photolithographic techniques to expose various underlying features on the board while masking others. During the soldering process the solder will deposit onto the exposed underlying components. It is necessary that the solder mask material be formulated such that it can be applied by the appropriate methods, for example curtain coating. Suitable photoimagable compositions including many that use epoxies are described in the following U.S. Pat. Nos. 4,279,985; 4,458,890; 4,351,708; 4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623; 4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197.
More recently an improved cationically photoimagable solder mask is described in U.S. Pat. No. 5,026,624 assigned to the assignee of the present application, disclosure of which is incorporated herein by reference. In fact U.S. Pat. No. 5,026,624 teaches an improved photoimagable cationically polymerizable epoxy based coating material. The material includes an epoxy resin system consisting essentially of between about 10% to about 80% by weight of a polyol resin which is a condensation product of epichlorohydrin and bisphenol A having a molecular weight of between about 40,000 and 130,000; between about 20% and about 90% by weight of an epoxidized octafunctional bisphenol A formaldehyde novolak resin having a molecular weight of 4,000 to 10,000; and between about 35% and 50% by weight of an epoxidized glycidyl ether of tetrabromo bisphenol A having a molecular weight of between about 600 and 2,500 if flame resistant properties are desired. To this resin system is added about 0.1 to about 15 parts by weight per 100 parts of resin of a cationic photoinitiator capable of initiating polymerization of said epoxidized resin system upon expose to actinic radiation; optionally a photosensitizer in an amount of up to about 10 parts by weight may be added.
The solder mask material is normally exposed to UV radiation from a medium pressure mercury lamp through a phototool which is opaque in the areas where the solder mask is to be removed. After exposure to UV radiation, the circuit boards are baked for a short time to accelerate the crosslinking reaction initiated by the sulfonium salt photolysis products. Bake temperatures between about 100.degree. C. and 150.degree. C. and times between about 2 and 10 minutes are used. An example of such formulations is given in U.S. Pat. No. 5,026,624 Table I.
In processing negative working resists, unexposed areas of the imaged film are typically removed from the surface of a printed circuit board or substrate by action of a liquid developer in a spray form for a duration of several minutes or less. Depending on the particular type of photoresist composition the liquid developer may be a simple organic solvent, an aqueous solution of an inorganic base, or as described in U.S. Pat. No. 3,475,171, a combination of organic solvent and aqueous base to form a semi-aqueous developer.
Methyl chloroform (MCF), a/k/a 1,1,1-trichloroethane, and methylene chloride (MC), a/k/a dichloromethane are solvents which are widely used in the electronic packaging art and in other arts for developing and removing a number of photoresists which are otherwise resistant to chemical attack. Highly alkaline electroless copper plating bath used in additive processes, for example, typically provides a harsh environment for photoresist. In general, the more chemically impervious resists are removable in an organic solvent such as methylene chloride. For less demanding chemical environments, aqueous developable photoresists may be adequate. The organically developable resists, however, continue to be used in an electroless copper environment and in the print band and thin film technologies in conjunction with acrylate-based resist such as DuPont's Riston T-168 and solvent process-able solder masks such as the DuPont Vacrel 700 and 900 series, environments in which the aqueous resists are vulnerable.
Use of 1,1,1-trichloroethane and methylene chloride is disfavored because of growing environmental concerns over the effect of gaseous halogenated hydrocarbons on the depletion of earth's ozone layer and concerns over introducing suspected carcinogens to the atmosphere. Several countries have set goals for their total elimination. However, there continue to be many manufacturing processes in which use of resists which are aqueously developable simply is not feasible.
The industry therefore continues to search for organic solvents as alternates to 1,1,1-trichloroethane and methylene chloride. The new solvents must meet specific manufacturing and environmental requirements with respect to flammability, toxicity, ability to effect dissolution, shelf-life, waste disposal, ability to recycle, simplicity of composition, and compatibility with a spectrum of resists.
Alternative solvents for stripping solvent based Riston photoresists are also described in IBM Technical Disclosure Bulletin, June 1989, p. 302, published anonymously. There have been previous attempts reported in the art to provide environmentally friendly alternative to 1,1,1-trichloroethane and methylene chloride.
The commonly assigned U.S. Pat. No. 5,268,260 of N. R. Bantu, Anilkumar Bhatt, Ashwindumar Bhatt, G. W. Jones, J. A. Kotylo, R. F. Woen, K. I. Papathomas, and A. K. Vardya for Photoresist Develop and Strip Solvents and Method for Their Use, incorporated herein by reference, describes the use of propylene carbonate, gamma-butyrolactone and benzyl alcohol as alternatives to halogenated hydrocarbon developers and strippers for use in developing and stripping acrylated-based photoresist such as DuPont Riston T-168 and solvent- processable solder masks such as the Varcel 700 and 900 series.
U.S. Pat. No. 5,268,260 describes developing and stripping the radiation-exposed resist in a high boiling solvent selected from the group consisting of propylene carbonate (PC) gamma butyrolactone (BLO) and benzyl alcohol (BA). The developing process occurs at about 25 to 45 degrees centigrade and the stripping process occurs at about 50 to 100 degrees centigrade.
At the end of each process the high boiling solvents, propylene carbonate, gamma-butyrolactone and benzyl alcohol, must be rinsed from the photoresist or solder mask with a compatible solvent or water. The effluents produced by these processes are impure solutions of propylene carbonate, gamma-butyrolactone and/or benzyl alcohol, laden with both dissolved and suspended photoresist or solder mask and other impurities. The commonly assigned U.S. Pat. No. 5,275,734 of J. A. Shurtleff, and K. P. Unger for Chemical Pre-Treatment and Biological Destruction of Propylene Carbonate Waste Streams Effluent Streams to Reduce the Biological Oxygen Demand Thereof describes a method of treating a process waste stream containing the non-biodegradable solvent propylene carbonate. However, none of the references provide a method for treating the waste streams which result from developing or stripping photoresists and solder masks with gamma-butyrolactone and/or benzyl alcohol.